1. Field of the Invention
The present invention relates to a vehicle body frame of a motorcycle, and particularly to a vehicle body frame suitable for an off-road type motorcycle.
2. Description of the Related Art
In general, as a method of molding a head pipe of a vehicle body frame, a method using casting and molding and a method using forging and molding have been known. In the casting and molding, the degree of design freedom is high, and structure members can correspond to a complex shape, but an internal defect such as a blowhole sometimes remains. On the other hand, in the forging and molding, the sufficient degree of design freedom cannot be obtained but a highly rigid and strong product having no internal defect can be obtained. For this reason, one having a high load to act on a head pipe such as an off-road type motorcycle, for example, is molded by the forging and molding.
Conventionally, as one in which a head pipe is molded by the forging and molding, one molded with right and left split molds has been known (see Patent Document 1, for example). As for a head pipe in Patent Document 1, a cylindrical part for supporting a steering shaft, and joint parts for a pair of right and left main frames and a down frame are formed simultaneously by right and left split molds. On the upper portion rear side of the cylindrical part, a vertical wall part extending outward (rearward) in a radial direction is provided, and the paired right and left main frames are joined to annular wall parts projecting in right and left directions from the above vertical wall part. The down frame is joined to a pair of tongue piece parts projecting from the lower portion rear side of the cylindrical part so as to face each other in up and down directions.
Further, as one in which a head pipe is molded by the forging and molding, one molded with front and rear split molds has also been known (see Patent Document 2, for example). As for a head pipe in Patent Document 2, a cylindrical part for supporting a steering shaft, and joint parts for a pair of right and left main frames and a down frame are formed simultaneously by front and rear split molds. The pair of right and left main frames is joined to an upper side tongue piece part projecting from the upper portion rear side of the cylindrical part so as to fit to upper wall portions of the main frames. The down frame is joined to a lower side tongue piece part projecting from the lower portion rear side of the cylindrical part so as to fit to a front wall portion of the down frame.    [Patent Document 1] Japanese Patent No. 4132431    [Patent Document 2] Japanese Patent No. 4233022
However, in the head pipe described in Patent Document 1, annular projecting parts project in the right and left directions perpendicular to the vertical wall part by the restriction of undercut of the right and left split molds. In the above case, the annular wall parts are joined to the main frames so that tips of the annular wall parts bump into side wall portions of the main frames. Due to changes in cross-sectional areas of the portions where the main frames and the annular wall parts bump into each other, the rigidity of a vehicle body frame changes suddenly, and thus there has been a problem that the stress concentration in the main frames cannot be alleviated.
Further, in the head pipe described in Patent Document 2, the upper side tongue piece part and the lower side tongue piece part that extend in forward and rearward directions are molded by the front and rear split molds, so that the depth of the molds is increased and a processing stroke at the time of forging is lengthened. For this reason, a large-scale processing machine is required and a processing time is prolonged. Further, a load to act on the molds is large, so that the lifetime of the molds is shortened. Thus, a problem that the molding cost of the head pipe increases has been caused. Further, the head pipe in Patent Documents 1 and 2 is subjected to the restriction by undercut of the molds, so that a problem that wastes are increased to thereby increase the weight of the whole vehicle body frame has also been caused.